Office lighting constitutes almost 30% of the electrical consumption in buildings. As the cost and energy efficiency of light-emitting-diode (LED) light sources improve, they are becoming viable alternatives for fluorescent lamps, further offering the advantage of color control. It is recognized that lighting control strategies based on occupant presence information are very effective in reducing energy consumption. For example, in unoccupied areas the illumination may be dimmed or extinguished. Hence the design of green buildings may benefit from presence-adaptive lighting control systems.
It is known that active sensors, such as ultrasound based sensors, provide better detection than passive infrared sensors in large volumetric spaces. In larger physical areas, it is commonplace to have multiple active presence sensors for proper detection coverage. It is further known that generally active sensors are more sensitive than passive infrared sensors. An ultrasonic array sensor has been described in WO 2011/151796 A1 for reliable presence sensing that, when interfaced with a lighting control system, provides reliable illumination rendering.
In “Sensor Scheduling for Target Tracking in Networks of Active Sensors,” Acta Automatica Sinica, November 2006, by Xiao et al. it is noted that one issue with wireless sensor networks of active sensors is the inter-sensor interference when nearby ultrasonic sensors work simultaneously. Such interference may result in sensor detection errors and should be dealt with properly. Inter-sensor interference also introduces technological constraints in design and implementation of wireless sensor networks. Sensor scheduling is used to avoid inter-sensor interference and implement collaboration between sensors. The network is synchronized and the time is divided into timeslots. The period for each slot should be larger than the die-out time of the ultrasonic wave in a ranging operation. In this paper, to avoid inter-sensor interference, sensors are scheduled such that during any timeslot only one sensor in an inter-sensor interference region can sense the target.